Rename x86_64-unknown-none-elf to x86_64-unknown-none

Most Rust freestanding/bare-metal targets use just `-unknown-none` here,
including aarch64-unknown-none, mipsel-unknown-none, and the BPF
targets. The *only* target using `-unknown-none-elf` is RISC-V.

The underlying toolchain doesn't care; LLVM accepts both `x86_64-unknown-none`
and `x86_64-unknown-none-elf`.

In addition, there's a long history of embedded x86 targets with varying
definitions for the `elf` suffix; on some of those embedded targets,
`elf` implied the inclusion of a C library based on newlib or similar.
Using `x86_64-unknown-none` avoids any potential ambiguity there.

(Work on this target sponsored by Profian.)

Author: joshtriplett

compiler/rustc_target/src/spec/mod.rs

@@ -944,7 +944,7 @@ supported_targets! {
 
     ("armv6k-nintendo-3ds", armv6k_nintendo_3ds),
 
-    ("x86_64-unknown-none-elf", x86_64_unknown_none_elf),
+    ("x86_64-unknown-none", x86_64_unknown_none),
 }
 
 /// Warnings encountered when parsing the target `json`.

compiler/rustc_target/src/spec/x86_64_unknown_none_elf.rs renamed to compiler/rustc_target/src/spec/x86_64_unknown_none.rs

@@ -280,7 +280,7 @@ target | std | host | notes
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 `x86_64-unknown-hermit` | ? |  |
 `x86_64-unknown-l4re-uclibc` | ? |  |
-`x86_64-unknown-none-elf` | * |  | Bare x86_64, softfloat
+`x86_64-unknown-none` | * |  | Freestanding/bare-metal x86_64, softfloat
 `x86_64-unknown-none-hermitkernel` | ? |  | HermitCore kernel
 `x86_64-unknown-none-linuxkernel` | * |  | Linux kernel modules
 `x86_64-unknown-openbsd` | ✓ | ✓ | 64-bit OpenBSD